(414b) Hydrodynamic Entrainment in Micro-Confined Suspensions and its Implications for Two-Point Microrheology

We study hydrodynamic entrainment in spherically confined colloidal suspensions of hydrodynamically interacting particles as a model system for intracellular and other micro-confined biophysical transport. Modeling of transport and rheology in such inhomogeneous soft materials requires an accurate description of the microscopic forces driving particle motion, such as entropic and hydrodynamic forces, and of particle interactions with nearby boundaries. Experimentally, information about the rheological properties of micro-confined biophysical systems is often measured via two-point microrheology, where it is assumed that particle entrainment exhibits the same qualitative features as that of particles in unbound domains; the effects of confining boundaries are thus neglected. In the present work, we carry out dynamic simulations of concentrated, spherically confined colloids as a model system to study the effect of 3D confinement on entrainment and rheology. We show that entrainment between two tracer particles exhibits qualitatively different functional dependence on inter-particle separation compared to an unbound suspension. We develop a scaling theory that collapses the concentrated mobility of spherically confined suspensions for all volume fractions and particle to cavity size ratios onto a master curve. For widely separated particles, the master curve can be predicted via a Green’s function, which suggests a framework with which to conduct two-point microrheology measurements near confining boundaries. The implications of these results for two-point microrheology experiments in micro-confined biophysical systems, such as the interior of eukaryotic cells, are discussed.